Pneumonia in children is a major cause of morbidity and the leading cause of death in children younger than 5 years of age worldwide. Among children in the United States, pneumonia and influenza are the eighth leading causes of death. Although the burden of pneumonia in children is well appreciated, recent data on the incidence and etiology of pneumonia in children are few. Thus far, determining the true etiology of pneumonia is complicated due to 1) the large number of bacterial and viral agents that may cause pneumonia;2) the frequency of viral and bacterial co-infections;3) the limited availability of deep respiratory specimens from children;4) the limitations of current laboratory technology and 5) antibiotic therapy initiated before microbiological evaluation. These limitations result in empirical clinical management of children with pneumonia. The optimum management of pediatric pneumonia is dependent on understanding the epidemiology of the pathogens that cause pneumonia by age and geographic region. The University of Utah, in collaboration with the Associated and Regional University Pathologists Laboratories and Research Enterprise, as well as Idaho Technology, Inc., is uniquely suited to address this problem. As part of a multicenter research study directed by the CDC, we are conducting a prospective study to identify viral and bacterial pathogens which cause community-acquired pneumonia and complicated bronchiolitis in hospitalized children. To determine the incidence and etiology of pediatric hospitalized pneumonia and to further understand the complications associated with influenza, we propose the following Specific Aims: Specific Aim 1: Identify all children in Salt Lake County diagnosed and hospitalized for clinically diagnosed and radiologically-confirmed CAP. Specific Aim 2: Determine the population-based incidence, etiology, and outcomes of pediatric influenza-associated pneumonia in Salt Lake County, including primary viral pneumonia, viral and bacterial co-infection, and post-influenza bacterial pneumonia. Specific Aim 3: Document the bacterial and viral etiology of pediatric pneumonia due to established and emerging pathogens using molecular analysis and high-order multiplex testing. Improve the ability to identify the etiology of presumed bacterial infection using molecular analysis of pleural fluid and blood. Quantify and compare respiratory viral loads in children hospitalized with pneumonia.